The [http://en.wikipedia.org/wiki/Hello_world_program "Hello World"] program is the traditional first program for many classes. Flashing an LED is the embedded equivalent. Here we will explore a few ways to flash and LED on the Beagle and explore General Purpose I/O (gpio) along the way. This will call be done from the command line of the Beagle, so there is no need for the host computer.

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The [http://en.wikipedia.org/wiki/Hello_world_program "Hello World"] program is the traditional first program for many classes. Flashing an LED is the embedded equivalent. Here we will explore a few ways to flash and LED on the Beagle and explore General Purpose I/O (gpio) along the way. These calls will be done from the command line of the Beagle, so there is no need for the host computer.

== gpio via the Shell Command Line and sysfs ==

== gpio via the Shell Command Line and sysfs ==

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[[File:Bone gpio.JPG|300px]]

[[File:Bone gpio.JPG|300px]]

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[[File:BoneGPIO.png|300px]]

So how do you know where to connect it? The BeagleBone [http://beagleboard.org/static/beaglebone/latest/Docs/Hardware/BONE_SRM.pdf System Reference Manual] has the details. Figure 3 on page 18 of RevA5.01 shows:

So how do you know where to connect it? The BeagleBone [http://beagleboard.org/static/beaglebone/latest/Docs/Hardware/BONE_SRM.pdf System Reference Manual] has the details. Figure 3 on page 18 of RevA5.01 shows:

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Notice '''gpio60''' has appeared. All we need to do is tell it which direction and then turn it on.

Notice '''gpio60''' has appeared. All we need to do is tell it which direction and then turn it on.

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beagle$ '''cd gpio60'''

beagle$ '''echo out > direction'''

beagle$ '''echo out > direction'''

beagle$ '''echo 1 > value'''

beagle$ '''echo 1 > value'''

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== Reading a switch - bone ==

== Reading a switch - bone ==

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Now that you have an LED working, wiring in a switch is easy. The picture above shows a push button switch wired in at the bottom of the P9 header. A 4.7k&Omega; resistor is attached to the '''-''' bus and the switch. The other end of the switch is attached to pin 42 which is '''gpio0_7'''.

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Now that you have an LED working, wiring in a switch is easy. The picture above shows a push button switch wired in at the bottom of the P9 header. Attach the '''+''' bus to one pole on the switch with a wire. The other end of the switch is attached to pin 42 which is '''gpio0_7'''.

Based on what you saw above, show how to read the switch.

Based on what you saw above, show how to read the switch.

Line 130:

Line 132:

Push the pushbutton and see what happens.

Push the pushbutton and see what happens.

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How can this work without a pulldown resistor? It turns out the Bone has an internal pulldown (and up) resistor that can be software enabled. Try:

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beagle$ '''cd /sys/kernel/debug/omap_mux'''

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beagle$ '''ls'''

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ain0 gpmc_ad2 lcd_data3 mii1_txd2

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ain1 gpmc_ad3 lcd_data4 mii1_txd3

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ain2 gpmc_ad4 lcd_data5 mii1_txen

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ain3 gpmc_ad5 lcd_data6 mmc0_clk

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ain4 gpmc_ad6 lcd_data7 mmc0_cmd

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ain5 gpmc_ad7 lcd_data8 mmc0_dat0

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ain6 gpmc_ad8 lcd_data9 mmc0_dat1

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ain7 gpmc_ad9 lcd_hsync mmc0_dat2

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board gpmc_advn_ale lcd_pclk mmc0_dat3

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ecap0_in_pwm0_out gpmc_ben0_cle lcd_vsync rmii1_refclk

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emu0 gpmc_ben1 mcasp0_aclkr spi0_cs0

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emu1 gpmc_clk mcasp0_aclkx spi0_cs1

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gpmc_a0 gpmc_csn0 mcasp0_ahclkr spi0_d0

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gpmc_a1 gpmc_csn1 mcasp0_ahclkx spi0_d1

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gpmc_a10 gpmc_csn2 mcasp0_axr0 spi0_sclk

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gpmc_a11 gpmc_csn3 mcasp0_axr1 uart0_ctsn

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gpmc_a2 gpmc_oen_ren mcasp0_fsr uart0_rtsn

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gpmc_a3 gpmc_wait0 mcasp0_fsx uart0_rxd

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gpmc_a4 gpmc_wen mdio_clk uart0_txd

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gpmc_a5 gpmc_wpn mdio_data uart1_ctsn

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gpmc_a6 i2c0_scl mii1_col uart1_rtsn

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gpmc_a7 i2c0_sda mii1_crs uart1_rxd

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gpmc_a8 lcd_ac_bias_en mii1_rxclk uart1_txd

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gpmc_a9 lcd_data0 mii1_rxd0 usb0_drvvbus

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gpmc_ad0 lcd_data1 mii1_rxd1 usb1_drvvbus

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gpmc_ad1 lcd_data10 mii1_rxd2 vrefn

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gpmc_ad10 lcd_data11 mii1_rxd3 vrefp

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gpmc_ad11 lcd_data12 mii1_rxdv xdma_event_intr0

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gpmc_ad12 lcd_data13 mii1_rxerr xdma_event_intr1

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gpmc_ad13 lcd_data14 mii1_txclk

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gpmc_ad14 lcd_data15 mii1_txd0

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gpmc_ad15 lcd_data2 mii1_txd1

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beagle$ '''ls | wc'''

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125 125 1220

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Here you will find a list of how all the pin muxes are set. There are some 125 of them!

It's set to be a PULLDOWN. Sectiion 9.2.2.2, Page 877 of the AM335x Technical Reference manual discusses Pull Selection and tells which bits do what. The '''0x0027''' is the code that says which MODE is used and how the pull up/down resistor is set. (There seems to be a mismatch between the manual and the bone. The manual says bit 3 of 0x0027 enables the pull up/down. Bit 3 isn’t set, but the pull down is working.)

Can you modify the scripts above to read the switch and turn the LED on and off?

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* Can you modify the scripts above to read the switch and turn the LED on and off?

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* Rewire your switch to work with a pullup resistor.

== Analog in - bone ==

== Analog in - bone ==

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(This is based on [http://www.gigamegablog.com/2012/01/22/beaglebone-coding-101-using-the-serial-and-analog-pins/ this].)

(This is based on [http://www.gigamegablog.com/2012/01/22/beaglebone-coding-101-using-the-serial-and-analog-pins/ this].)

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The bone has eight Analog Inputs. Several are exposed on P9. They are labeled '''AIN''' in table 11 above. How many to you find?

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The bone has eight Analog Inputs. Several are exposed on P9. They are labeled '''AIN''' in table 11 above. How many do you find?

The AIN pins are sampled at 12 bits and 100k samples per second. The input voltage is between 0 and 1.8V. Fortunately, both voltages are available on P9.

The AIN pins are sampled at 12 bits and 100k samples per second. The input voltage is between 0 and 1.8V. Fortunately, both voltages are available on P9.

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beagle$ '''find / -name ain5'''

beagle$ '''find / -name ain5'''

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/sys/devices/platform/omap/tsc/ain6

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/sys/devices/platform/omap/tsc/ain5

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/sys/kernel/debug/omap_mux/ain6

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/sys/kernel/debug/omap_mux/ain5

'''^C'''

'''^C'''

I used '''Ctrl-C''' to interrupt since it already found what I wanted.

I used '''Ctrl-C''' to interrupt since it already found what I wanted.

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beagle$ '''cd /sys/devices/platform/omap/tsc'''

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beagle$ '''cd /sys/devices/platform/tsc'''

beagle$ '''ls'''

beagle$ '''ls'''

ain1 ain3 ain5 ain7 driver power uevent

ain1 ain3 ain5 ain7 driver power uevent

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The first argument tells which gpio port to toggle, the second tells how long to delay between toggling. In this example 0.05 s is 50 ms, which should give a period around 100ms. Measure the signal on an oscilloscope.

The first argument tells which gpio port to toggle, the second tells how long to delay between toggling. In this example 0.05 s is 50 ms, which should give a period around 100ms. Measure the signal on an oscilloscope.

Modify togglegpio (call it toggleLED) to toggle the LEDs. Can you get the LED to appear to dim by changing the duty cycle of the toggling?

Modify togglegpio (call it toggleLED) to toggle the LEDs. Can you get the LED to appear to dim by changing the duty cycle of the toggling?

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==== User Button to gpio 130 ====

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=== User Button to gpio 130 ===

Write a shell script that reads the User Button and outputs it value on gpio pin 130.

Write a shell script that reads the User Button and outputs it value on gpio pin 130.

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==== Count the User Button Presses ====

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=== Count the User Button Presses ===

Write a shell script that displays a count of the number of times the User Button has been pressed.

Write a shell script that displays a count of the number of times the User Button has been pressed.

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==== Copy gpio 130 to gpio 131 ====

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=== Copy gpio 130 to gpio 131 ===

Write a shell script that copies the value of gpio pin 130 to gpio pin 131. How much CPU time does it take? What's the delay from the time the input changes until the output changes? How constant is the delay?

Write a shell script that copies the value of gpio pin 130 to gpio pin 131. How much CPU time does it take? What's the delay from the time the input changes until the output changes? How constant is the delay?

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== Additional Exercises ==

=== Reading the Keyboard and Mouse ===

=== Reading the Keyboard and Mouse ===

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beagle$ '''cd /sys/class/input'''

beagle$ '''cd /sys/class/input'''

beagle$ '''ls -F'''

beagle$ '''ls -F'''

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beagle$ '''evtest event2'''

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beagle$ '''evtest /dev/input/event2'''

Hit ctrl-C to stop

Hit ctrl-C to stop

Revision as of 08:18, 18 September 2012

The "Hello World" program is the traditional first program for many classes. Flashing an LED is the embedded equivalent. Here we will explore a few ways to flash and LED on the Beagle and explore General Purpose I/O (gpio) along the way. These calls will be done from the command line of the Beagle, so there is no need for the host computer.

gpio via the Shell Command Line and sysfs

The easiest way to do general purpose I/O (gpio) on the Beagle is through a terminal window and a shell prompt. In Linux most everything is treated as a file. Even things that aren't files. In our class we'll use a virtual file system called sysfs. sysfs exposes the drivers for the hardware so you get easily use them.

Adding your own LED - bone

It's not hard to use the gpio pins to control your own LED. All you need is an LED and a 220Ω resistor. Here's a picture of how it's wired. We are just doing the LED at the top of the breadboard for now.

So how do you know where to connect it? The BeagleBone System Reference Manual has the details. Figure 3 on page 18 of RevA5.01 shows:

There at two expansion headers, P8 and P9. Look at the bottom of the left header and you'll see it's labeled P9. Table 11 on page 59 shows the pinout for P9.

On the first photo above you can see that pin 1 (Ground) is wired to the - bus and pin 3 (3.3V) is wired to the + bus. The 220Ω resistor is wired to the - bus and the other end is attached to the negative lead of the LED. The positive lead is attached to pin 12 which, as shown in Table 11, is attached to gpio1_28. The gpio pins are in banks of 32 each, so to find the gpio number to use on the Beagle, use 1*32+28=60. Here's how you turn it on

Notice gpio60 has appeared. All we need to do is tell it which direction and then turn it on.

beagle$ cd gpio60
beagle$ echo out > direction
beagle$ echo 1 > value

Your LED should be on! When you are done you can unexport the pin and it will disappear.

beagle$ cd ..
beagle$ echo 60 > unexport

Reading a switch - bone

Now that you have an LED working, wiring in a switch is easy. The picture above shows a push button switch wired in at the bottom of the P9 header. Attach the + bus to one pole on the switch with a wire. The other end of the switch is attached to pin 42 which is gpio0_7.

Based on what you saw above, show how to read the switch.

Once you have the switch and LED working you can use the following scripts to play with them.

beagle$ cd ~/exercises/gpio
beagle$ ./togglegpio.sh 60 0.1

The LED should be blinking on and off.

beagle$ ./readgpio.sh 7

Push the pushbutton and see what happens.

How can this work without a pulldown resistor? It turns out the Bone has an internal pulldown (and up) resistor that can be software enabled. Try:

It's set to be a PULLDOWN. Sectiion 9.2.2.2, Page 877 of the AM335x Technical Reference manual discusses Pull Selection and tells which bits do what. The 0x0027 is the code that says which MODE is used and how the pull up/down resistor is set. (There seems to be a mismatch between the manual and the bone. The manual says bit 3 of 0x0027 enables the pull up/down. Bit 3 isn’t set, but the pull down is working.)

Reading a gpio pin with an Oscilloscope - xM

Unfortunately the gpio pins don't appear here. It turns out the processor has more internal I/O lines than it has physical pins. Each physical pin can can be connected to up to 8 internal lines. BeagleBoardPinMux does a nice job of explaining it all. The big clue is here BeagleBoardPinMux#Beagle which references Table 22 on page 108 of the -xM System Reference Manual.

Note that gpio130 appears on pin 21 of the Expansion Header. Also note that pins 27 and 28 are ground. Attach your scope probe to these. Now, let's put a signal on the pin.

Note that if you are the root user (which is the default case for Angstrom), you will have to type the following command:

beagle$ ./togglegpio 130 0.05

The first argument tells which gpio port to toggle, the second tells how long to delay between toggling. In this example 0.05 s is 50 ms, which should give a period around 100ms. Measure the signal on an oscilloscope.

Assignment: gpio from the shell

Measuring a gpio pin on an Oscilloscope

Answer the following questions about gpio measurements.

What's the min and max voltage?

What period is it?

How close is it to 100ms?

Why do they differ?

Run htop and see how much processor you are using.

Try different values for the sleep time (2nd argument). What's the shortest period you can get? Make a table of the values you try and the corresponding period and processor usage.

How stable is the period?

Try launching something like mplayer. How stable is the period?

Try cleaning up togglegpio and removing unneeded lines. Does it impact the period?

togglegpio uses bash (first line in file). Try using sh. Is the period shorter?

What's the shortest period you can get?

Toggling the LEDs

Modify togglegpio (call it toggleLED) to toggle the LEDs. Can you get the LED to appear to dim by changing the duty cycle of the toggling?

User Button to gpio 130

Write a shell script that reads the User Button and outputs it value on gpio pin 130.

Count the User Button Presses

Write a shell script that displays a count of the number of times the User Button has been pressed.

Copy gpio 130 to gpio 131

Write a shell script that copies the value of gpio pin 130 to gpio pin 131. How much CPU time does it take? What's the delay from the time the input changes until the output changes? How constant is the delay?